1. Field of the Invention
The present invention relates generally to a beam index color cathode ray tube, and more particularly to a beam index color cathode ray tube which can reproduce color with high fidelity.
2. Description of the Prior Art
In a prior art beam index color cathode ray tube, a single electron beam scans a target screen which consists of triads of parallel red, green and blue vertical color phosphor stripes sequentially repeating across the screen. The color phosphor stripes are sequentially scanned by a scanning electron beam which crosses the color phosphor stripes horizontally in sequence from one side of the screen to the other. Index phosphor stripes are provided on the inner surface of the screen parallel to and in known relationship to the color phosphor stripes. As the electron beam scans horizontally across the screen, it excites the index phosphor stripes into producing a light signal behind the screen. Such light signal is detected by a photodetector to produce an index signal which has a known relationship to the instantaneous position of the electron beam on the screen.
The index signal is used to control the modulation of the electron beam such that the electron beam is density modulated with the red primary color signal when the beam scans across a red phosphor stripe, with the green primary color signal when the beam scans across a green phosphor stripe and with the blue primary color signal when the beam scans across a blue phosphor stripe, respectively.
FIGS. 1-3 show arrangements of index phosphor stripes on the inner surface of the screen in prior art color cathode ray tubes. In FIG. 1, the pitch P.sub.I of the index phosphor stripes 1a is the same as, or an integral multiple of, the pitch P.sub.T of each triad of red, green and blue color phosphor stripes R, G and B. The positional relationship between the index phosphor stripes 1a and the respective triads of red, green and blue color phosphor stripes R, G and B is fixed and readily determined from the index signal and hence there is no need to provide a special circuit which establishes synchronization of the color modulation by the index signal and accordingly a simple circuit is sufficient.
Since the positional relation between the index phosphor stripes 1a and the red, green and blue phosphor stripes R, G and B is fixed, any phase shift in the index signal produced, for example, by color modulation of the scanning beam results in faulty color synchronization and degrades color reproduction fidelity. This is especially noticeable in the reproduction of highly saturated color since the high electron beam current for a particular highly saturated color creates an apparent shift in phase of the index signal.
For the above reason, a one-to-one relationship between index phosphor stripes 1a and color phosphor stripe triads is not desirable. Therefore, the arrangements in FIGS. 2 and 3 are used. The pitch P.sub.I of the index phosphor stripes 1a is selected to be a non-integral multiple of the pitch P.sub.T of the triads of color phosphor stripes. Thus, pitches P.sub.I of 2/3, 4/3 or generally 3n.+-.1)/3 (where n is 0, 1, 2, - - - ) of the pitch P.sub.T of the triads of red, green and blue color phosphor stripes R, G and B may be used. With the foregoing arrangement, the positional relationships between the index phosphor stripes 1a and the triads of red, green and blue color phosphor stripes R, G and B are varied sequentially so that a phase shift in an index signal due to a reproduced color signal does not appear uniformly across the screen and hence the color reproduction is achieved with higher fidelity.
With the latter arrangement, however, the variation in positional relationships across the screen between the index phosphor stripes 1a and the triads of red, green and blue color phosphor stripes R, G and B, requires a synchronization technique to establish color synchronization.
One way to establish color synchronization is to provide a means for determining when the electron beam begins scanning across the color phosphor stripes and to thereafter keep a running count of the index phosphor stripes that are scanned or crossed. A method of accomplishing this is shown in FIG. 4. The index phosphor stripes 1a are provided across the image area of the screen in non-integral spaced relationship with the triads of color phosphor stripes R, G and B, as previously described, and, in addition, a set of synchronizing index phosphor stripes 1b are provided on the beam scanning run-in or marginal area outside the image area of the screen. In order to distinguish between the run-in area and the image area, and thus to be able to discern the transition from one area to the other, the pitch of the index phosphor stripes 1b in the run-in area is made suitably larger, for example, three times, the pitch of the index phosphor stripes 1a in the viewing or image area. The index signal obtained when the beam scans the index phosphor stripes 1b in the run-in area is used to establish color synchronization. However, since a plurality of phosphor stripes 1b are necessary in the run-in area and the pitch of the index phosphor stripes 1b is large, the width of the marginal portion of the screen must also be large when using the described scheme according to the prior art. In addition, if noise is mixed with the index signal from the run-in area, the noise can sometimes occur in positions which simulate the index signal from the viewing area. When this happens, color synchronization is displaced and incorrect colors are reproduced.